1. Field of the Invention
The present invention relates to a torque converter and, in particular, to a torque converter used in an automatic transmission of a motor vehicle.
2. Description of Related Art
A conventional torque converter used in an automatic transmission is provided with a pump, a turbine and a stator located between the pump and turbine. The pump and turbine each has a plurality of blades and a core which supports the inner ends of the blades. As shown in Japanese Patent Laid-Open Publication No. 60-260765, in the conventional torque converter, the pump core projects slightly beyond the radial outer edges of the pump blades toward the turbine and the turbine core projects slightly beyond the radial outer edges of the turbine blades toward the pump.
The capacity coefficient K.sub.P representing the performance of the torque converter is given by the following equation: EQU K.sub.P =T.sub.IN /(N.sub.E /1000).sup.2
where T.sub.IN is is the torque applied to an input shaft of the torque converter and N.sub.E is the rotational speed of the input shaft of the torque converter or the engine speed. As shown in the above equation, the capacity coefficient K.sub.P represents a relationship between T.sub.IN and N.sub.E, and it will be understood that for one and the same speed N.sub.E of the input shaft of the torque converter, the larger the capacity coefficient K.sub.P is, the larger the torque T.sub.IN applied to the torque converter will be.
Hydraulic fluid contained in the torque converter is caused to fly outward along the core of the pump by centrifugal force. The fluid flows into the turbine from the pump, generates a torque to rotate the turbine, and returns to the pump through the stator. By the circulation of the hydraulic fluid described above, the torque converter can transmit torque from the pump to the turbine, namely from the engine to the automatic transmission. The larger the velocity of the circulation of the hydraulic fluid is, the larger the transmitted torque or capacity coefficient K.sub.P is. On the other hand, the smaller the velocity of the circulation of the hydraulic fluid is, the smaller the transmitted torque or the capacity coefficient K.sub.P is.
The velocity of the hydraulic fluid becomes large when the velocity ratio between the turbine velocity and the pump velocity is small, and decreases as the velocity ratio increases.
Therefore, the capacity coefficient K.sub.P is large in regions where the speed ratio is small, such as at and near the idling region, and becomes smaller as the velocity ratio becomes larger or the engine speed becomes higher.
The capacity coefficient K.sub.P can be controlled by controlling the velocity of the circulation of the hydraulic fluid. If the capacity coefficient K.sub.P could be decreased at and near the idling region by controlling the velocity of the circulation of the hydraulic fluid, fuel consumption could be improved in such regions. It might be throught that this could be achieved by, for example, a torque converter in which the pump and the turbine are provided with blades having projections on their surfaces so as to decrease the capacity coefficient K.sub.P in such regions.
However, a torque converter of this type could be disadvantageous in that the capacity coefficient K.sub.P in regions where the speed ratio is large, namely, in the driving ( running ) region, would decrease in comparison with the conventional torque converter with no projections on the surfaces of the blades and, therefore, acceleration performance would decrease in the driving region.